A number of integrated circuit microprocessor chips have been implemented using advanced MOS LSI (metal oxide semiconductor large scale integrated circuit) manufacturing processes, including the P-channel and the faster, more sophisticated N-channel technologies. The microprocessor chips are the heart of complete micro-computer systems which additionally typically include a number of power supplies, external RAM (random access memory) devices, ROM (read only memory) devices and a variety of buffer interface and peripheral control devices. A relatively small number of semiconductor chips are required to implement a complete microcomputer system. A simple but complete microcomputer using such microprocessor packages may be implemented on a small printed circuit card, requiring as few as four or five integrated circuit packages. Some control functions may be implemented with even fewer integrated circuit packages. Thereby, the power of computer data processing is made available at very low cost for use in a wide variety of industrial and communications equipment, such as in process and manufacturing control systems, computer peripheral and terminal hardware, parameter-control systems of all types from microcomputers in the automobile to the control systems for traffic and anywhere else that random logic computer control needs optimizing. The savings in design time and cost for the microcomputer systems designer is greatly reduced by the availability of such MOS LSI microprocessor and memory and peripheral units. Printed circuit board layouts are simplified. The complex interconnections required for large numbers of conventional integrated circuits are replaced by ROMs. The only interconnect wiring on printed circuit cards runs between the various address and data buses and input/output devices. The cost savings are not limited to direct component costs, but extend also to other related system hardware offer the user advantages in using a particular microprocessor unit. Now families of support circuits coupleable directly to a bidirectional data bus, which characterizes mos of the known microprocessor chips, and the new microprocessor units (MPU's) are fare more powerful, generally, than the earliest first generation MOS LSI microprocessor chips in that the instruction sets for the newer devices are much larger and more powerful (the most advanced microprocessor chips include about 70 instructions) and the instruction execution times are about an order of magnitude faster. Yet, further improvements in overall system performance may be based on improvements in chip architecture of the microprocessor chips and new systems architectures encompassing the MPU chips and associated peripheral circuit families.
Known microprocessor chips have had an input for receiving an external interrupt signal.